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POTASSIUM FERRICYANIDE                             85

30 c.c. of hydrochloric acid (D 1-12) and treated in the hot with barium
chloride, should give no turbidity or precipitate, even after standing.

2.  Chlorides.—i gram dissolved in water and acidified with nitric
acid, should not be rendered turbid by silver nitrate.

3.  Lime, Magnesia.—2 grams dissolved in 30 c.c. of water and mixed
with 10 c.c. of ammonia (D = 0-96) should be rendered turbid neither by
ammonium oxalate nor by sodium phosphate.

4.  Sodium Salts.—These are detected by the flame, after moistening
with hydrochloric acid.    Indirectly they are indicated by the content of
chromic anhydride (see 5).

5.  Determination of the Dichromate.—This can be carried out in
two ways :

(a)  BY REDUCTION.    5 grams of the dichromate are dissolved in water
and the volume made up to i litre.    10 c.c. of this solution (= 0-05 gram
of substance) are acidified with sulphuric acid, mixed with about i gram
(weighed exactly) of ferrous ammonium sulphate, the excess of the ferrous
salt remaining unaltered being then titrated with N/io-permanganate.
The difference between the quantity of iron used, which is given by (grams
of ferrous ammonium sulphate) ~ 7, and that of the iron remaining un-
changed (i c.c. N/io-permanganate = 0-0056 gram Fe) gives the amount
of iron required to reduce the 0-05 gram of substance taken : Fe x 0-8781 =
K2Cr207 and Fe X 0-5969 = Cr03.

(b)  IODOMETRICALLY.    5 grams of dichromate are dissolved to i litre.
To 25 c.c. of the solution (0-125 gram of substance) are added 4-5 grams
of potassium iodide, 20 c.c. of 50% sulphuric acid and about half a litre
of water, the iodine liberated being titrated with N/io-sodium thiosulphate
in presence of starch paste,    i c.c. N/io-thiosulphate = 0-0049 gram of
K2CraO7 or 0-00333 gram of   Cr03.

When the proportion of Cr03 found is greater than the theoretical pro-
portion for pure potassium, dichromate (68%), the presence of sodium
dichromate is to be inferred.

Commercial potassium dichromate is usually guaranteed to contain 67-5-
68% of CrO3.

POTASSIUM   FERRICYANIDE (Red Prussiate of Potash)

K6Fea(CN)12 = 658-6

Reddish-brown, anhydrous crystals soluble in water, insoluble in alcohol.
It may contain the same impurities as the ferrocyanide, these being detect-
able similarly (see succeeding article). It may also contain ferrocyanide,
detectable as follows :

1.   Ferrocyanide.—The i : 20 solution is treated with a few drops of
dilute ferric chloride : if the red prussiate is pure only a faint brown colora-
tion appears, whereas in presence of ferrocyanide a blue coloration or
precipitate is formed.

2.  Quantitative Determination.—The ferricyanide is first reduced
to ferrocyanide and this then estimated.   2 grams of the salt are dissolvedss, it reacts with tin* sodium chloride